1. Field of the Invention
The present invention relates in general to machining apparatus and process wherein a cold gas stream or blast is supplied to a heat-generating point or machining point such as a grinding point, and more particularly to cooling device and method using a cold gas stream.
2. Discussion of the Related Art
A machining operation on a workpiece is generally performed such that a machining tool in contact with the workpiece is moved relative to the workpiece while the workpiece is supported by suitable means. To reduce or minimize a temperature rise at the machining point due to heat generation as a result of machining of the workpiece by the machining tool, a machining apparatus is usually equipped with a cooling device.
An example of a conventional machining apparatus in the form of a centerless grinder equipped with a grinding liquid supply device is schematically illustrated in FIG. 7.
The centerless grinder includes a stationary work rest blade 1, a regulating wheel 2, and a grinding wheel 4. A workpiece 3 to be ground is interposed between the regulating and grinding wheels 2, 4. The regulating wheel 2 cooperates with the work rest blade 1 to support the workpiece 3 such that the workpiece is rotatable about its axis. In operation of the centerless grinder, the regulating wheel 2 is rotated in one direction indicated at "a" in FIG. 7, so that the workpiece 3 is rotated in a direction indicated at "b", in frictional contact with the regulating wheel 2. The portion of the workpiece 3 to be ground may have a cylindrical, tapered, conical or any other configuration having a circular shape in transverse cross section (as seen in the plane of FIG. 7).
The grinding wheel 4 is rotated in a direction indicated at "c", and is adapted to contact with the workpiece 3 at its circumferential grinding surface, so that the selected portion of the workpiece 3 is ground by the grinding wheel 4. While the circumferential position of instantaneous contact of the grinding wheel 4 with the workpiece 3 is indicated at point "g" in FIG. 5, the instantaneous contact between the grinding wheel 4 and the workpiece 3 takes place along a straight line, which extends in the axial direction of the workpiece 3 (grinding wheel 4), where the portion of the workpiece 3 to be ground is cylindrical. The length of this straight line is equal to the axial dimension of the circumferential grinding surface of the grinding wheel 4. Although the instantaneous contact is indicated by the grinding point or line "g", the contact actually occurs in some surface areas of the grinding wheel 4 and the workpiece 3 as viewed in the circumferential direction.
The rotating speeds of the regulating wheel 2 and the grinding wheel 4 are determined such that the peripheral speed of the grinding wheel 4 rotated in the direction "c" is higher than the peripheral speed of the workpiece 3 rotated by frictional contact with the regulating wheel 2 in the direction "b". Consequently, the circumferential grinding surface of the grinding wheel 4 is moved relative to the circumferential surface of the workpiece 3, whereby the portion of the workpiece 3 in contact with the grinding wheel 4 is ground. In this grinding operation, the regulating wheel 2 functions to regulate the rotating speed of the workpiece 3, while applying a certain degree of braking to the workpiece 3 rotated by frictional contact with the grinding wheel 4.
In FIG. 7, reference character "H" represents a center height of the workpiece 3 with respect to the regulating and grinding wheels 2, 4. More specifically described, the center height H is a distance between the center or axis of rotation of the workpiece 3 and the centers or axes of rotation of the regulating and grinding wheels 2, 4. To permit an intended centerless grinding operation on the workpiece 3, the center height H must be adequately adjusted. However, the optimum center height H more or less varies depending upon the various grinding conditions. In this respect, it is not easy to properly adjust the center height, and the adjustment requires a high level of knowledge and skill in the centerless grinding operation.
Qualitatively, there is a tendency that an excessively small amount of the center height H results in deteriorated roundness of the ground portion of the workpiece 3, while an excessively large amount of the center height H results in instability of the workpiece 3 resting on the work rest blade 1, causing chatter marks in the form of flower leaves to be formed on the ground surface of the workpiece 3.
For increased stability of the workpiece 3 resting on the work rest blade 1, a work holder 8 may be provided in sliding contact with the workpiece 3, as indicated by one-dot chain line in FIG. 7, such that the points of contact of the workpiece 3 with the work rest blade 1 and the work holder 8 are located on the opposite sides of the grinding point g in the circumference direction of the workpiece 3.
Grinding heat is generated at and near the grinding point g. A temperature rise at the grinding point g due to the heat generation may cause burning or cracking of the ground surface of the workpiece 3, deterioration of the grinding accuracy due to a difference in the thermal expansion coefficient, and other problems. To avoid these problems, a grinding liquid nozzle 7 is provided above the grinding point g, so that a grinding liquid pumped up from a liquid tank 5 by a pump 6 is delivered through the nozzle 7 to the grinding point g and its vicinity.
The grinding liquid, which may be called a coolant, has not only a cooling function but also lubricating and cleaning functions. Lubrication at the grinding point g is important to prevent deterioration of the surface smoothness or finish of the ground portion of the workpiece 3. Further, the grinding liquid is effective to remove particulate foreign matters such as abrasive grains derived from the grinding wheel 4, for maintaining good grinding condition at the grinding point g.
The grinding wheel 4 must have a desired shape established by a truing operation wherein high spots on the wheel are removed so as to shape the wheel as needed. Since the grinding wheel 4 becomes dull or glazed during use, the grinding wheel 4 is required to be subjected to a dressing operation wherein dull grains of the wheel are removed to expose sharp cutting edges for improving the grinding capability of the grinding wheel 4. While the truing and dressing operations have distinct original purposes, these operations cannot and need not be clearly distinguished from each other.
The truing and dressing operations are performed by a dresser 9, which may be of a rotary type as indicated in FIG. 7 by way of example, or alternatively of a rod or blade type having a dressing head at one of its opposite ends. Before a grinding operation of the centerless grinder, the dresser 9 is brought to its retracted position as indicated by arrow "e" in FIG. 7. When the truing or dressing operation is required to be performed, the dresser 9 is moved to its advanced position as indicated by arrow "f". In the truing or dressing operation, too, heat is generated. To reduce the temperature rise at the point of contact between the dresser 9 and the grinding wheel 4, another grinding liquid nozzle 11 is disposed near the truing or dressing point of the grinding wheel 4, so that the grinding liquid is delivered to the truing or dressing point, as indicated at "h". To this end, the conduit between the pump 6 and the nozzle 11 is provided with a switch valve 10 for delivering the grinding liquid selectively to the grinding point g and the truing or dressing point.
The conventional centerless grinding apparatus equipped with the grinding liquid supply device as shown in FIG. 7 is capable of grinding the selected portion (e.g., roughly finished portion) of the workpiece 3 with high accuracy and efficiency. However, the grinding liquid supply device requires recirculating means for returning the used liquid back to the tank 5, for recirculation of the liquid, and also requires filtering means for cleaning the liquid and removing the metal particles and abrasive grains contained in the liquid. The recirculating and filtering means increase the cost of manufacture of the liquid supply device and the cost of operation of the centerless grinding apparatus.
The grinding liquid supply device suffers from another drawback that the grinding liquid is consumed and must be replenished from time to time, leading to a further increase in the cost of operation of the centerless grinding apparatus.
In the light of the above drawbacks, one of the present co-inventors et al. has proposed the use of a cold air cooling technique as disclosed in an article entitled "PRODUCTION MACHINING TECHNOLOGY FOR HARMONIZATION WITH ENVIRONMENT, Cutting and Grinding Techniques Using Cold Gas for Protecting Environment, and Realization of ISO 1400", journal entitled "MACHINES AND TOOLS" published Nov. 1, 1996 by Kogyo Chosakai, Tokyo, Japan. The cold gas cooling technique disclosed in this article uses a stream or blast of a cold gas whose temperature is about -30.degree. C. In this technique, it is desired to consider the following aspects:
(a) relationship between the temperature and amount of a cold air stream and the cooling capacity; PA1 (b) relationship between the temperature at the grinding point and the burning and cracking of the ground surface (mainly in the field of material science); and PA1 (c) relationship between the material of the grinding wheel and the amount of heat generation and temperature rise.
Although the cold air cooling technique has an advantage of eliminating the need of using a machining liquid such as a grinding liquid, the present inventors felt a need of improving the cold gas cooling efficiency, without increasing the structural complexity of the machining apparatus.